Discovery of Sex Linkage Using Drosophila
Thomas Hunt Morgan In the early 20th century, geneticist Thomas Hunt Morgan, skeptical of Mendel's gene hypothesis began testing the hypothesis with Drosophila melanogaster more commonly known as fruit flies. Due to a lack of test subjects, Morgan and his students captured wild fruit flies using buckets of rotting fruit hung on trees. Because theses wild flies were almost always of the same phenotype he referred to them as "wild type". This term is now used to indicate the most common phenotype in a population. Morgan used Drosophila because of their simple needs and fast reproduction rate. The life cycle of Drosophila is between 12 and 14 days and roughly 25 to 30 generations can be raised in a year. Using this, Morgan raised many generations of Drosophila, ''allowing him to study the phenotypic variants that popped up as well as analyze test crosses between the differntt phenotypes. X-Linked Inheritance Nettie Stevens, a biologist did a study on ''T. molitor ''(mealworms) in which she noticed a difference in size of the chromosomes contained in somatic cells. Females posses two large chromosomes while males possess one large and one small chromosome. Stevens dubbed the large chromosome X and the small chromosome Y and formed the hypothesis that the presence of two X's denoted a female and the presence of an X and a Y denoted a male. In 1910, Morgan performed a series of experiments that ended up validating Stevens' findings. Morgan's Experiment The whole experiment began when Morgan's wife, Lilian, found a white-eyed male ''Drosophila in a bottle with all wild-type red eyes. In order to decipher the cause of the mutant, Morgan and his team performed a series of test crosses. First a wild-type red-eyed female was crossed with the mutant white-eyed male, producing a F1 generation with predominantly red eyes, indicating this to be the dominant phenotype. Next the F1 was crossed with each other producing a F2 that had a 3:1 ratio of red to white eyes. In the F2, 2459 red-eyed females, 1011 red-eyed males and 728 white-eyed males were produced but not a single white-eyed female was present in the F2 generation. This indicated to morgan that white eyes must be linked to the male sex in some way. Shocked by this discovery, Morgan began performing new test crosses to investigate the mystery. First he crossed the original white-eyed male with one of his red-eyed daughters to produce an F1 generation with roughly equal amounts of red-eyed and white-eyed males and females. The final cross performed was reciprocal between a white-eyed female and a wild-type red-eyed male, this F1 contained red-eyed females and white-eyed males while the F2 was made up of equal frequencies of red-eyed and white-eyed males and females. Based on the test crosses performed, Morgan realized that the conveyance of the X chromosomes could explain the appearance of white and red eyes. To test the theory that the allele for eye color was linked to the X chromosome, Morgan dubbed the wild-type allele for red eyes "w+" and the allele for white eyes "w". In the first cross, the X chromosome of the white-eyed male was a w+ while the X chromosomes of the females had the alleles w and w+, producing red eyes from the dominant w+ allele. The F1 cross from this generation produced red-eyed females with w+/w and red-eyed males with w+/Y. The F2 of this cross produced identical proportions of white-eyed males with w/Y and red-eyed males with w+/Y. Based on these observations, Morgan was able to conclude and prove that genes can be linked to the X chromosome and thus present differently in differing sexes. He called this phenomenon X-linkage, more commonly known now as sex-linkage. References Mark F. Sanders, J. L. B. (2012). Genetic Analysis: An Integrated Approach: Library of Congress Cataloging-in-Publication Data 2012.